The present invention relates to a microstrip antenna of two-frequency separate-feeding type for circularly polarized waves which is employed for various radio communications.
A microstrip antenna is of wide application as an antenna for various communications, because it has a planar structure of a thickness sufficiently small as compared with the wavelength used and is lightweight. With a phased array antenna using a plurality of such microstrip antennas it is possible to electrically change a beam of radio wave by controlling the phase shift amount of a phase shifter connected to each antenna element. Such a phased array antenna features its thin, small and lightweight structure, and hence is expected to be applied to mobile communication and the like.
As is well-known in the art, the microstrip antenna is narrow-band. For example, assuming that a voltage standing wave ratio of the antenna, i.e. a criterion upon which to determine whether or not the antenna can be put to practical use, is 2 or below, the bandwidth of the microstrip antenna which satisfies the ratio is as small as several percents with respect to the center frequency, though it depends on the characteristic of a dielectric plate used. This means that an ordinary microstrip antenna cannot be used for communications in which transmit and receive radio waves higher than such a bandwidth as mentioned above. To solve this problem, microstrip antennas of various structures have been proposed so far.
However, conventional art has defects such as complicated structure and difficulty in fabrication.